1. Field of the Invention
The present is concerned with new wood-synthetic resin composite products and extrusion processes for forming these products.
2. Description of the Prior Art
Although wood is a naturally reproducible resource, the demand for wood is consistently high. Furthermore, the supply of good wood for construction purposes is beginning to diminish. Accordingly, there is an increasing urgency to find alternative sources of wood. One possible alternate source is through the production of artificial wood from a mixture of ingredients including recycled wood scraps such as wood meal, wood chips, saw dust, and newspapers, which are each by-products of industrial wastes and other industries using natural wood products.
Composite materials consisting of recycled wood scraps and a thermoplastic material have been known for many years. Generally, these composites are formed so that they may be used in many of the same applications as an all-wood product while offering advantages such as high resistance to rot, insects, and moisture. These products can have the same workability as wood and are splinter-free. However, these composites have not been successfully used as a direct replacement for wood.
Forming a wood-polymer composite into a final product has been accomplished using most of the techniques used for forming all-polymer products, including extrusion. While the technology for extruding all-polymer products is well developed with fairly predictable results, the extrusion of a wood-thermoplastic composite material using recycled input materials is subject to a much wider variance in the molecular makeup and physical characteristics of the input materials, depending upon available resources of the recycled material. Moreover, a wood-thermoplastic composite has unique melt flow characteristics which prevent the literal translation of polymer extrusion techniques for use in composite material extrusion.
Previous extruded wood-polymer composites have generally comprised polyethylene as the polymer. While extrusion processes are the preferred method for forming these composites due to the ease with which large quantities of the composite can be readily manufactured, it has previously been impossible to form wood-polypropylene composites by counter-rotating, twin screw extrusion processes in a commercially feasible manner. That is, manufacturers were unable to develop process conditions which properly processed large quantities of polypropylene without damaging the wood. Thus, there is a need for processes for extruding polypropylene and wood or other cellulosic materials to yield suitable composite products.
The present invention fills this need by broadly providing wood-synthetic resin composite products and methods of forming such products by extrusion processes.
In more detail, the products are formed by introducing ingredients including respective quantities of a fibrous or cellulosic material and polypropylene into the inlet of an extruder (preferably a twin screw extruder). Preferably, the weigh blender is positioned immediately above the extruder, at the extruder inlet, so that the blend of ingredients is formed immediately prior to entering the extruder, thus minimizing or preventing separation of the ingredients. This is not the case with prior art processes which convey the ingredients to, or xe2x80x9ccramxe2x80x9d the ingredients into, the extruder inlet so as to cause separation of the ingredients and yield an inferior composite product-The screw(s) is then rotated at a rate of from about 10-50 rpm, and preferably from about 15-34 rpm to advance the ingredients through the extruder barrel and out the extrusion die to form the composite product. Preferably, the screw(s) has a compression ratio of from about 2:1 to about 4:1, and more preferably from about 2.8:1 to about 3.6:1.
The temperature of the ingredients in the extruder barrel is preferably from about 150-260xc2x0 C., and more preferably from about 175-230xc2x0 C. The retention time of the ingredients in the barrel should be from about 20-120 seconds, and more preferably from about 40-80 seconds. Finally, the ingredients should be advanced through the barrel at a rate of from about 500-2,000 lbs/hr., and more preferably from about 1,000-1,500 lbs/hr.
The fibrous material is preferably present in the ingredients at a level of from about 20-80% by weight, more preferably from about 30-70% by weight, and even more preferably from about 50-70% by weight, based upon the total weight of the ingredients taken as 100% by weight. The polypropylene is preferably present in the ingredients at a level of from about 20-80% by weight, more preferably from about 30-70% by weight, and even more preferably from about 30-50% by weight, based upon the total weight of the ingredients taken as 100% by weight.
Preferred fibrous materials include those selected from the group consisting of sawdust, newspaper, alfalfa, wheat pulp, wood scraps (e.g., ground wood, wood flour, wood flakes, wood chips, wood fibers, wood particles), wood veneers, wood laminates, cardboard, straw, cotton, rice hulls, paper, coconut shells, peanut shells, bagasse, plant fibers, bamboo fiber, palm fiber, kenaf, and mixtures thereof. Furthermore, the average particle size of the fibrous material should be less than about xc2xd inch, and more preferably from about {fraction (1/16)}-xc2xc inch. Finally, the particles of the fibrous material should have an average aspect ratio (i.e., the ratio of the length to the widest thickness) of at least about 10:1, preferably at least about 20:1, and more preferably from about 30:1 to about 50:1. The use of such long particles increases the flexural modulus of the product as compared to products with lower aspect ratios by at least about 25%, and preferably at least about 40%, thus causing the final composite product to have a stiffness comparable to natural wood.
The preferred polypropylene for use in the invention is reactor flake polypropylene (i.e., the polymer flakes as they are produced in the reactor), preferably without any further treatment (e.g., without the addition of chemical additives or modifiers) to the polypropylene. The preferred polypropylene has a melt index at 230xc2x0 C. of from about 0-10 g/10 min., preferably from about 0.1-4 g/10 min., and more preferably from about 0.1-1 g/10 min. Furthermore, it is preferred that the polypropylene has a bulk density of from about 20-40 lbs/ft3, and more preferably from about 28-32 lbs/ft3. The average fiber length or particle size of the polypropylene flakes utilized should be from about 350-1,000 xcexcm, and preferably from about 500-700 xcexcm.
The resulting composite product is in the form of a self-sustaining body and has an ASTM D-6109 flexural modulus of from about 600-1,100 psi/1000 (i.e. 600,000-1,100,000 psi), and preferably from about 800-1,100 psi/1,000 (i.e. 800,000-1,100,000 psi). The product should have an actual density of from about 40-60 lbs/ft3, and preferably from about 50-58 lbs/ft3.
A number of optional ingredients can also be added to modify or adjust the properties of the final composite product. Examples of such ingredients include acrylic process aids (e.g., Rohm and Haas K175, Kaneka Kane-Ace PA-101), UV stabilizers (e.g., CYTEC 38535, CYTEC 3346), and coloring agents. If a process aid is utilized, it is preferably present in the ingredients at a level of from about 0.5-5% by weight, and more preferably from about 1-2% by weight, based upon the total weight of the ingredients taken as 100% by weight. Unexpectedly, these acrylic process aids are particularly useful in the present invention in spite of the fact that they are intended to be used in PVC products rather than polypropylene products.
It will be appreciated that the inventive method allows for the formation of high-strength, high-stiffness composite products having properties which greatly resemble the properties of natural wood. Furthermore, the inventive composite products are much stiffer than prior art polyethylene-cellulosic fiber products due to the fact that polypropylene is used. The products can be used in a wide number of areas including door sills and jambs, fascia board, window edging, window sills, decorative architectural trim (e.g., deck or patio railing), simulated hardwood flooring, and landscaping products (e.g., raised bed edging, flowerbed edging, driveway edging).